Serum levels of ampicillin and gentamycin in neonates of varying gestational age.

Serum concentrations of gentamycin and ampicillin were investigated at loading and in steady state in two groups of neonates, of 26-33 weeks and 34-40 weeks gestation. At loading the usual intravenous dose of gentamycin (2.5 mg/kg) was increased by 50%, the usual intravenous dose of ampicillin (50 mg/kg) by 100%. Gentamycin and ampicillin were administered subsequently at the same intervals, 12 h in the 34-40 weeks group, and 18 h in the 26-33 weeks group. Adequate serum levels were achieved from the first day of treatment. For practical reasons ampicillin and gentamycin can be administered subsequently at identical intervals also when the intervals exceed 12 h. The ideal dosing interval for gentamycin in very preterm neonates is 24 h. When treating small preterm and term neonates with an aminoglycoside, rapid serum concentration analyses should be available, and the treatment modified accordingly.